Chalcedony is a compact, microcrystalline form of silica known for its smooth texture, waxy lustre and wide range of colours. Many familiar jewellery stones—including agate, onyx, carnelian, sardonyx, bloodstone and blue lace agate—belong to the chalcedony family.

This guide explores chalcedony’s composition, hardness, formation and physical properties. It also explains how its best-known varieties differ and why names such as agate, jasper and onyx are sometimes used inconsistently in the gemstone trade.

What is chalcedony?

Chalcedony is a microcrystalline to cryptocrystalline form of silica. Instead of growing as large individual crystals visible to the eye, it consists of extremely small interlocking mineral grains and fibres.

This fine structure gives chalcedony its dense appearance. Polished pieces often look smooth, slightly translucent and waxy rather than displaying the glass-like transparency and visible crystal faces associated with clear quartz.

The word chalcedony can be used in two ways:

• As a broad mineralogical term covering many microcrystalline silica materials, including agate, onyx and carnelian
• As a narrower trade name for unbanded, translucent material that is commonly white, grey or blue

This difference in usage explains why one source may describe agate as a variety of chalcedony while another places “chalcedony” and “agate” in separate retail categories.

If you are interested in its traditional symbolism, read our separate guide to chalcedony meaning and uses.

Is chalcedony quartz?

Chalcedony belongs to the broader quartz and silica family, but its internal structure differs from the large visible crystals of amethyst, rose quartz and clear quartz.

Macrocrystalline quartz forms individual crystals that may be large enough to see clearly. Chalcedony consists of microscopic intergrowths that are too small to distinguish without specialised equipment.

Both materials are composed primarily of silicon dioxide, written as SiO₂. Their different crystal habits produce differences in appearance, transparency and texture.

Compare chalcedony with other quartz varieties in our guides to:

• amethyst properties
• rose quartz properties
• clear quartz properties

Chalcedony composition and structure

Chalcedony is composed mainly of microscopic quartz and may also contain moganite. Quartz and moganite are both forms of silicon dioxide, but their atoms are arranged differently.

The proportion of moganite varies between specimens and may change as the material ages geologically. For this reason, chalcedony is more accurately described as a microcrystalline silica material containing intergrown quartz and moganite rather than simply “tiny quartz crystals.”

Its main structural characteristics are:

• primarily composed of silicon dioxide
• microcrystalline or cryptocrystalline texture
• commonly contains intergrown quartz and moganite
• forms dense masses, nodules, crusts and cavity fillings
• may contain pigments, inclusions and layered structures

Small amounts of iron compounds, manganese minerals, chlorite and other inclusions can influence its colour and pattern.

What is the hardness of chalcedony?

Chalcedony has a Mohs hardness of approximately 6.5–7. It is therefore close in hardness to other quartz-family materials such as amethyst, rose quartz and clear quartz.

This hardness makes chalcedony suitable for:

• beads and bracelets
• pendants and earrings
• cabochons and rings
• carvings and cameos
• tumbled stones
• decorative objects

Hardness measures resistance to scratching, not resistance to all damage. Chalcedony can still chip or fracture when dropped or struck against a hard surface.

Physical properties of chalcedony

• Mineral family: quartz and silica
• Chemical composition: primarily SiO₂
• Structure: microcrystalline to cryptocrystalline
• Mohs hardness: approximately 6.5–7
• Lustre: waxy, dull or glass-like when highly polished
• Transparency: transparent at thin edges, translucent or opaque
• Cleavage: none
• Fracture: commonly conchoidal or uneven
• Common form: nodules, masses, crusts and cavity fillings

Chalcedony’s dense structure allows it to take a smooth polish. Its lack of cleavage is useful in jewellery and carving, although irregular fractures and internal cavities may still create weaker areas.

How does chalcedony form?

Chalcedony commonly forms when silica-rich water enters cavities, cracks and pores in rock. As conditions change, the dissolved or gel-like silica is deposited in extremely fine layers and gradually solidifies.

This process often takes place at relatively low temperatures and may occur in:

• cavities within volcanic rocks
• fractures and veins
• sedimentary rocks
• fossil cavities and pore spaces
• areas affected by silica-rich groundwater or hydrothermal fluids

Repeated episodes of deposition can create bands, colour changes and complex internal patterns. Uniform deposition may form relatively even chalcedony, while rhythmic layers can produce agate.

Chalcedony forms very differently from obsidian. Obsidian is volcanic glass produced when silica-rich molten rock cools rapidly, whereas chalcedony grows gradually from silica-bearing fluids or gels.

Why does chalcedony form inside geodes?

A geode begins with an empty or partly empty cavity in rock. Silica-bearing water enters the space and deposits mineral material along its walls.

Chalcedony or agate may form the outer layers first. If space remains, larger quartz crystals can later grow towards the centre of the cavity. This is why some geodes have a banded chalcedony shell surrounding visible quartz points.

Not every geode develops in exactly the same way. The final result depends on available silica, trace minerals, temperature, pressure and the number of deposition stages.

Chalcedony colours

Chalcedony occurs in an extensive range of colours, including:

• white and cream
• grey and black
• pale to deep blue
• orange, red and reddish brown
• green
• yellow and brown
• multicoloured and banded combinations

Pure microcrystalline silica is colourless or pale. Most colours are produced by microscopic inclusions, mineral pigments, structural effects or gemstone treatments.

Iron oxides and hydroxides commonly create yellow, orange, red and brown shades. Chlorite and related minerals can contribute green patterns, while fine inclusions and light scattering may produce pale blue chalcedony.

Chalcedony, agate and jasper: what is the difference?

Chalcedony, agate and jasper are closely related, and the boundaries between the names are not always applied consistently.

• Chalcedony: the broad microcrystalline silica material; in retail, the name often refers to unbanded translucent material
• Agate: translucent to opaque chalcedony characterised primarily by bands or other organised patterns
• Jasper: generally opaque microcrystalline silica coloured by abundant mineral inclusions

Some specimens contain both translucent agate areas and opaque jasper areas. Trade names are often based on appearance, locality or historical use rather than a perfectly strict mineralogical boundary.

Agate: banded chalcedony

Agate is a patterned variety of chalcedony best known for its bands, rings and layered structures. Its patterns form as silica is deposited in repeated stages inside a cavity or fracture.

The bands may be straight, curved, concentric, angular or irregular. Differences in trace elements, inclusions and crystal orientation create contrasting colours and levels of transparency.

Agate is available in many forms, including fortification agate, dendritic agate, Botswana agate and blue lace agate.

Blue lace agate: pale blue banded chalcedony

Blue lace agate is a pale blue to blue-grey variety of agate known for white, cream or deeper blue bands. Its fine patterns may resemble lace, waves or narrow clouds.

Mineralogically, blue lace agate belongs to the same chalcedony family as other agates. The word “lace” describes its delicate pattern rather than a separate mineral species.

In crystal traditions, blue lace agate is associated with thoughtful communication and composure. These meanings are symbolic rather than mineral properties.

Onyx: parallel-banded chalcedony

Onyx is a chalcedony variety defined mineralogically by straight, parallel bands. Traditional examples may contain black and white, brown and white or other contrasting layers.

Completely black material sold as black onyx is often chalcedony that has been dyed to produce a deeper and more consistent colour. Dyeing is an established treatment, but it should be disclosed when known.

Black onyx is especially popular in beads, signet rings, pendants and minimalist jewellery. Read more about its symbolism, treatments and care in our guide to black onyx meaning.

Carnelian: orange and red chalcedony

Carnelian is a translucent orange, red or brownish-orange variety of chalcedony. Its colour comes primarily from iron compounds within the silica.

Heat can deepen or modify carnelian’s colour, and some commercial material begins as paler chalcedony or agate that has been heated or dyed.

Carnelian has been used for beads, seals and carved jewellery for thousands of years. In crystal traditions, it is associated with courage, motivation and creativity.

Read more in our guide to carnelian meaning and uses.

Sard: brownish-red chalcedony

Sard is a brown, reddish-brown or dark orange chalcedony variety. It is closely related to carnelian, and the distinction between the two is based mainly on colour.

Carnelian is generally brighter and more orange-red, while sard is usually darker and browner. Because colour grades into one another, some specimens may be described by either name.

Sardonyx: layered sard and onyx

Sardonyx is a banded chalcedony variety containing layers of sard together with white, black or differently coloured onyx-type bands.

Its contrasting layers have made it especially suitable for cameos, seals and intaglios. A carver can remove material selectively so that different parts of an image appear in different coloured layers.

Sardonyx has been used in decorative objects and jewellery since antiquity. Its name reflects the combination of sard and onyx.

Bloodstone: green chalcedony with red spots

Bloodstone, also known as heliotrope, is an opaque dark green chalcedony or jasper traditionally recognised by its red spots.

The green colour is commonly related to chlorite or similar inclusions, while the red markings are generally caused by iron oxides such as hematite.

Bloodstone demonstrates how abundant inclusions can transform the colour and opacity of microcrystalline silica while the material remains part of the broad chalcedony family.

Moss agate: chalcedony with branching inclusions

Moss agate is translucent chalcedony containing green, brown, red or black mineral inclusions that resemble moss, branches or miniature landscapes.

It does not contain plant matter. The patterns are created by minerals such as chlorite, hornblende and iron or manganese compounds.

Moss agate usually lacks the regular bands associated with classic agate, but its established trade name refers to its botanical-looking patterns.

Learn more in our guide to moss agate meaning and properties.

Other chalcedony varieties

The chalcedony family includes many other materials distinguished by colour, pattern, inclusions or transparency.

• Chrysoprase: translucent apple-green chalcedony coloured primarily by nickel-related compounds
• Prase: green chalcedony coloured by mineral inclusions
• Dendritic agate: pale chalcedony containing branching iron or manganese oxide patterns
• Plasma: dark green, often opaque chalcedony containing mineral inclusions
• Chrome chalcedony: green chalcedony coloured by chromium
• Fire agate: layered chalcedony displaying iridescent colours caused by thin iron-oxide layers

Trade names vary between regions and sellers. A single specimen may sometimes fit more than one descriptive category.

Chalcedony compared with macrocrystalline quartz

Chalcedony and visible-crystal quartz have the same principal chemical component but differ in texture and growth.

Property	Chalcedony	Macrocrystalline quartz
Crystal size	Microscopic	Often visible to the eye
Typical appearance	Dense, smooth and waxy	Glass-like with identifiable crystal faces
Composition	Intergrown quartz and commonly moganite	Primarily crystalline quartz
Common forms	Nodules, bands, crusts and cavity fillings	Crystal points, clusters and massive material
Examples	Agate, onyx and carnelian	Amethyst, clear quartz and smoky quartz

Both groups have a hardness near 7, but their textures produce clearly different visual characteristics.

Chalcedony treatments

Chalcedony is porous enough at a microscopic level to accept certain treatments, and its colour has been modified for centuries.

Common treatments include:

• Dyeing: used to create or intensify black, blue, green, red and other colours
• Heating: commonly used to deepen orange and red shades in carnelian-type material
• Bleaching: may be used before another colour treatment
• Impregnation: occasionally used to improve the appearance of porous or fractured material

Treated chalcedony remains chalcedony, but the treatment is relevant to its value, care and accurate description. Sellers should disclose known treatments.

How to identify chalcedony

Chalcedony is usually recognised through a combination of appearance, hardness, density, lustre and internal structure. Visual inspection alone may not identify every variety or treatment.

Common characteristics include:

• a dense texture without visible individual crystals
• a waxy to glass-like polished surface
• translucency around thin edges
• bands, clouds, dendrites or relatively even colour
• a hardness sufficient to resist scratching from ordinary metal objects
• conchoidal fractures when broken

Glass imitations may contain round bubbles, moulding seams or a texture that looks unusually uniform. Resin and plastic are typically lighter and warmer to the touch, but these observations are not definitive tests.

Gemmological equipment may be needed to distinguish chalcedony reliably from glass, quartzite, opal and treated imitations.

Chalcedony in jewellery and carvings

Chalcedony has been used in jewellery, seals and carvings for thousands of years. Its dense structure allows it to take a smooth polish and preserve fine carved details.

Common chalcedony products include:

• round and faceted beads
• cabochons
• cameos and intaglios
• signet-ring inserts
• pendants and earrings
• tumbled stones
• small carvings and decorative objects

Translucent chalcedony can appear different against various skin tones, metals and backgrounds. Banded varieties may also be cut deliberately to place a particular pattern at the centre of the finished piece.

Explore chalcedony stones and jewellery

How to clean and care for chalcedony

Chalcedony is relatively durable and generally straightforward to maintain.

• Clean it briefly with lukewarm water and mild soap.
• Use a clean, soft cloth or brush.
• Rinse and dry the stone thoroughly.
• Avoid bleach, acids and strong household cleaners.
• Protect it from hard impacts and sudden temperature changes.
• Store jewellery separately to preserve its polish.
• Avoid prolonged intense sunlight when the stone is dyed.
• Do not use steam or ultrasonic cleaning when treatment, fractures or settings are uncertain.

Solid untreated chalcedony can normally tolerate brief contact with water. Prolonged soaking is unnecessary and may be unsuitable for dyed stones, porous specimens or jewellery containing adhesives.

Traditional meanings of chalcedony varieties

Mineral properties and crystal meanings are different subjects. The composition and hardness of chalcedony can be examined scientifically, while its spiritual associations come from cultural traditions and modern crystal practices.

Chalcedony is broadly associated with balance and thoughtful communication, but individual varieties have developed their own symbolic themes:

• Black onyx: protection, grounding and inner strength
• Carnelian: courage, motivation and creativity
• Blue lace agate: composed communication and reflection
• Moss agate: growth, patience and connection with the outdoors
• Bloodstone: courage, determination and endurance

These meanings are symbolic and are not scientifically demonstrated effects.

Key chalcedony properties at a glance

• Chalcedony belongs to the quartz and silica family.
• It consists of microscopic intergrowths of quartz and commonly moganite.
• Its chemical composition is primarily silicon dioxide.
• Its Mohs hardness is approximately 6.5–7.
• It commonly forms in cavities, fractures and porous rock.
• Agate, onyx, carnelian, sardonyx and blue lace agate are chalcedony varieties.
• Bloodstone and many jaspers belong to the broader microcrystalline silica group.
• Chalcedony is widely used in jewellery, beads, carvings and tumbled stones.

Frequently asked questions about chalcedony

Is chalcedony quartz?

Yes. Chalcedony is a microcrystalline to cryptocrystalline silica material composed mainly of intergrown quartz and commonly moganite.

What is the chemical formula of chalcedony?

Chalcedony is composed primarily of silicon dioxide, SiO₂. It may also contain small amounts of other minerals and elements responsible for its colour and pattern.

What is the hardness of chalcedony?

Chalcedony has a Mohs hardness of approximately 6.5–7, making it suitable for many types of jewellery and decorative objects.

What does chalcedony look like?

Chalcedony usually has a dense, fine-grained appearance and a waxy to glass-like polish. It can be translucent or opaque and occurs in many colours and patterns.

How does chalcedony form?

Chalcedony commonly forms when silica-rich water or gel enters cavities, fractures and pores in rock. The silica is gradually deposited as extremely fine intergrown mineral material.

What stones belong to the chalcedony family?

Common chalcedony varieties include agate, onyx, carnelian, sard, sardonyx, blue lace agate, moss agate, chrysoprase and bloodstone.

Is agate chalcedony?

Yes. Agate is a chalcedony variety identified primarily by its bands or other organised internal patterns.

Is onyx chalcedony?

Yes. Mineralogical onyx is parallel-banded chalcedony. Much of the uniformly black material used in jewellery is dyed chalcedony.

Is carnelian chalcedony?

Yes. Carnelian is translucent orange, red or brownish-orange chalcedony coloured mainly by iron compounds.

Is bloodstone chalcedony?

Bloodstone is commonly classified as opaque green chalcedony or jasper containing red iron-oxide spots. It belongs to the broader microcrystalline silica group.

What is the difference between agate and jasper?

Agate is generally translucent to opaque and characterised by banding. Jasper is generally opaque and contains more abundant mineral inclusions. Some specimens contain characteristics of both.

Can chalcedony be dyed?

Yes. Chalcedony is frequently dyed to create or intensify colours. Black onyx and brightly coloured agates are common examples of treated material.

Can chalcedony go in water?

Solid chalcedony can normally be cleaned briefly with lukewarm water. Avoid prolonged soaking when the stone is dyed, fractured, porous or set with adhesive.

Is chalcedony suitable for everyday jewellery?

Yes. Its hardness and dense structure make chalcedony suitable for regular jewellery use. It should still be protected from strong impacts and harsh chemicals.

Summary

Chalcedony is a microcrystalline silica material composed mainly of intergrown quartz and moganite. Its dense structure, extensive colour range and Mohs hardness of approximately 6.5–7 make it an important material for jewellery, carvings and decorative objects.

Agate, onyx, carnelian, sardonyx, moss agate, blue lace agate and bloodstone all belong to or are closely associated with the chalcedony family. Their different colours and patterns result from variations in structure, inclusions, trace elements and treatments.

Explore chalcedony stones and jewellery